In a sheet-fed offset printing machine, a printing plate is fastened on a plate cylinder by means of a clamping rail allocated to the leading edge of the printing plate and a clamping rail allocated to the trailing edge. An example of such a printing machine may be seen in U.S. Pat. No. 5,511,478 (DE 43 39 344 C1) which discloses a device for the automatic changing of printing plates in a printing machine. Here, a printing plate, which is to be inserted, has certain built in guards to stop movement and a guide device to assist its contact with the cylinder and insertion into the clamping rail of the edges. After this plate has been inserted into the clamping rail and the plate's leading edge has been clamped, the plate is drawn onto the cylinder by the latter being rotated forwards. During insertion into the leading edge clamping rail and the drawing process, the central or rear part of the printing plate still makes contact with the guard.
To assure high print quality, printing plates are usually fitted to the plate cylinder in a precise location using a register system. For example, U.S. Pat. No. 5,383,402 (EP 0 551 976 A1) discloses a register system for mounting a plate on a plate cylinder that includes a plate lockup device, reference pins and a lamp. The plate lockup device is provided in a gap formed in the circumference surface of the plate cylinder. The reference pins are electrically rendered conductive by contacting an insertion end of a plate inserted into the plate lockup device, thereby detecting insertion of the plate. The lamp confirms and indicates insertion of the plate from an output from the reference pins thereby allowing a manual installation of a printing plate to visually verified.
While the register system disclosed in this patent does work for its intended purpose, it does suffer disadvantages. For example, the register system suffers the disadvantage of requiring extra space for the contact areas on the cylinder and for the rollers on the frame. Additionally, these rollers also increase outlay on construction. Still further, any soiling of the contact areas or the contacts/rollers could result in an erroneous signal transmission.